The practice has made known step-wise automatic transmissions, which are constructed, at least in the planetary transmission manner or are designed as auxiliary transmissions. Step-wise automatic transmissions, which are constructed in planetary transmission design are equipped with a hydro-dynamic torque converter as a starting element and with frictional shifting means so that, with this type of transmission, shifting can be carried out without an interruption in continuous traction. So that the motor torque in the drive string, which is conducted to the output drive shaft, can be transmitted by the frictional shifting element, it is necessary that these be relatively large in dimensioning whereby, however disadvantageously, in the shifting element, draw-power losses when towing or losses in the open state of the shifting element are large. Since the frictional shifting elements are generally mostly hydraulically operated, a large and complex hydraulic system with a hydraulic pumping system is necessary which, because of a pick-up capacity of the pump arrangement and by way of leakage losses, this leads to a deterioration of the efficiency of the transmission. The draw-power losses can be reduced by the insertion of a substitution for the frictional shifting element, whereby the gear-step exchange would then be carried out, but disadvantageously and with the occurrence of traction interruptions.
In order to execute traction interruption-free shifting with automatic step-wise transmissions, even of the auxiliary type, then these must be constructed as the so-called dual-clutch transmissions. This type of transmission possesses two frictional shifting elements by way of which, in the transmission, two power strings are shifted into the power flow of the transmission or, alternately, are shifted out of the power flow. In the two power strings, shape-fit, shifting element gear pairings of various ratios, have the capability of the shifting “into” or “out of”. In this case, the ratios of the power strings can then be adjusted or changed, if the currently employed power string is not precisely shifted into the power flow, but rather finds itself in a load-free condition. In the area of the employed, shape-fit, shifting elements, in comparison to the frictional shifting elements, there appears the essentially smaller traction torque of a planetary transmission so that, in this case, losses of lesser magnitude occur and a total transmission efficiency in a small assembly is enhanced.
The dual-clutch transmissions, which are made in the auxiliary-transmission method are, in comparison to planetary gear transmissions, disadvantageous because of being characterized by a lower power density wherein, for example, in regard to the same gear stage, they require more construction space, which space, especially in the case of motor vehicles, has a very limited availability.
Further, an effective gear efficiency of an auxiliary transmission lies somewhat at the lower level of hand-shifting, which is caused by the known dual spur gear engagement. Contrary to this, planetary gear transmissions possess an efficiency, which is governed by the currently involved gear set scheme. This is true especially in the principal gears for driving and the transmission efficiency is higher than in the case of a coaxially designed auxiliary transmission.
DE 31 31 138 A1 discloses a changeable transmission based on a load carrying, planetary gear. This is intended for motor vehicles with automatic transmission control and having a plurality of interlocked, planetary gear sets. The vehicles are known to have shifting clutches and shifting brakes for the formation of drive strings with different ratios as well, in which only two shifting clutches which, for example, are installed on the motor drive shaft as load shifting clutches and which, as directed, principally transmit drive torque over two drive strings. The remaining shifting clutches and shifting brakes can then always be shifted if they are not already included in the transfer of the torque. By means of appropriate selection of and the arrangement of the coupled planetary gear sets, shifting clutches and shifting brakes in combination with the two load shifting clutches, there arises a high multiple usage of the individual construction elements as well as gaining a simple assembly with satisfactory ratios and a comparative number of gear positions.
As disclosed by DE 31 31 138 A1, the known, load-shiftable planetary transmission, however, exhibits the disadvantage, that the shape-fit, shifting elements are controlled throughout by the rotating components of the planetary transmission, whereby a faultless functionality is attained only objectionably because of a very high technically complex manufacture with resulting undesirable high fabrication costs.
The present invention has the purpose of making a planetary transmission available, which is of low cost in its manufacture.
Within the field of the invention, this purpose is achieved with a planetary transmission.